## Real Analysis Exchange

### Jensen’s Inequality for Conditional Expectations in Banach Spaces

August M. Zapała

#### Abstract

In this note we present a simple proof of the inequality $\Phi \left( E^{\mathcal{A}}\xi \right) \leq E^{\mathcal{A}}\Phi (\xi )$ a.s. for separable random elements $\xi \mathcal{I}n L_{1}(\Omega ,\mathcal{F},P;X)$ in a Banach space $X,$ where $E^{\mathcal{A}}\left(\cdot\right)$ denotes conditional expectation with respect to the $\sigma$-field $\mathcal{A} \subset \mathcal{F}$, and $\Phi :X\rightarrow \mathbb{R}$ is a convex functional satisfying certain additional assumptions which are less restrictive than known till now. Some consequences of the above result are also discussed; e.g., it is shown that if $\xi$ is a Gaussian random element in $X$, then there exists a constant $0<c< \infty$ such that for each $\sigma$-field $\mathcal{A}_{0}\subset \mathcal{F}$ the family $\left\{ \exp \{c\left\| E^{\mathcal{A}}\xi \right\| ^{2}\}\mathcal{A}_{0}\subseteq \mathcal{A} \subseteq \mathcal{F}\right\}$ is uniformly integrable.

#### Article information

Source
Real Anal. Exchange, Volume 26, Number 2 (2000), 541-552.

Dates
First available in Project Euclid: 27 June 2008

https://projecteuclid.org/euclid.rae/1214571348

Mathematical Reviews number (MathSciNet)
MR1844134

Zentralblatt MATH identifier
1010.60020

#### Citation

Zapała, August M. Jensen’s Inequality for Conditional Expectations in Banach Spaces. Real Anal. Exchange 26 (2000), no. 2, 541--552. https://projecteuclid.org/euclid.rae/1214571348

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